This invention relates generally to acoustic well logging of earth boreholes, and in particular, to methods and apparatus for measuring various formation parameters associated with the generation and reception of acoustic waves.
In a characteristic acoustic waveform, the compressional wave arrives at the acoustic receiver first because of the velocity of a shear wave being substantially less than that of the compressional wave velocity, typically being approximately 60% thereof. Furthermore, the amplitude of the shear wave on arrival is oftentimes much greater than the amplitude of the compressional wave and upon arriving, will even cause an addition of the two waves upon the coincidence of positive arrivals. In the prior art systems, with a gain of the amplifier set at an appropriate level for detecting the compressional wave first arrival, the first arrival of the shear wave will place the system in saturation. Conversely, with a gain of the amplifier set for the detection of the shear wave first arrival, the amplifier gain for the compressional wave will be too small for accurate measurement. Thus, there is no ideal gain setting for both the compressional and the shear wave in accordance with the heretofore state of the art.
It is therefore the primary object of the present invention to provide new and improved methods and apparatus for acoustically logging the formations surrounding earth boreholes.
It is another object of the invention to provide new and improved method and apparatus for detecting compressional and shear acoustic waves originating from the same transmitted acoustic wave.
The objects of the invention are accomplished, generally, by methods and apparatus which vary the gain of a subsurface amplifier in an acoustic well logging instrument used for amplifying subsequently received cycles in the same wave in response to the detection of a predetermined parameter relating to a given half cycle or to the cross-over between half cycles.